A dynamic light-scattering measuring method has already become an established technology, and a measuring device that measures a particle size of a fine particle by utilizing the method is widely used. This method has advantages including a small number of necessary parameters, nondestructive measurement, and easy-to-use handling. The dynamic light-scattering measuring method realizes the highly accurate measurement under an environment in which a single scattering theory, that is, a theory that the light scattered once is detectable without scattering another particle again can be applied in principle. For example, the particle size is accurately measured in a dilute solution containing the particle of 0.01% or less. However, depending on the particle size and a refractive index, when the particle containing solution has a high concentration to be not able to ignore an influence of multiple scattering, that is, a phenomenon that the light scattered once is scattered again by another particle, a dynamic particle characteristic that is necessary for the conventional measuring devices based on the single scattering theory differs from actual one.
FIG. 5 is an explanatory view illustrating modeling of the light-scattering phenomenon due to a difference in particle concentration. Generally, in a diluted system, it can be assumed that a particle 73 does freely Brownian motion and the scattering of light 71 occurs only once (see FIG. 5(a)). On the other hand, in a concentrated system, at least one-time scattering is mixed (see FIG. 5(b)). Therefore, the measurement based on the single scattering cannot be performed because the phenomenon is matched with neither the theory nor the actual measurement.
Recently, there is reported the result in which a time correlation function and a power spectrum of the scattering light are measured using a Michelson interferometer as a dynamic light-scattering measuring apparatus 80 (see FIG. 6). In the Michelson interferometer, a low-coherence light source is used (see JP-A-2003-106979 (“JP-A” means unexamined published Japanese patent application) and JP-A-2005-121600). In FIG. 6, the dynamic light-scattering measuring apparatus 80 includes a low-coherence light source 87, a lens 88, optical fibers 89a to 89d, a photocoupler 91, a collimator 81, a mirror 82, an oscillating element (piezoelectric oscillator) 83, a sample cell (scattering medium) 84, a collector 85, and a spectrum analyzer (detector) 86. The use of the apparatus 80 in which the Michelson interferometer is utilized can extract only a scattering light component from a region that is specified substantially equal to an optical path length of reference light, so that only a single scattering component of the scattering light can selectively be detected from a high-concentration medium. The spectrum and time correlation function of the scattering light fluctuating over time are obtained based on the single scattering component, which allows the measurement of the dynamic characteristic of the high-concentration medium particle.